The present invention relates to the field of diffraction optics, in particular to the design, production, and use of high efficiency diffraction gratings.
The term “grating” refers to a periodic or semi-periodic array of lines on a surface. A diffraction grating operates on the principle that phases of light traveling through different light paths, reflected or transmitted by the grating lines, constructively interfere in certain directions while destructively interfering in other directions. The angular directions with constructive interference form the so called diffraction orders and are known to depend on wavelength of the light and pitch of the grating. There is virtually no light in the angular directions between these orders since the contributions interfere here destructively. Such gratings have various uses in the field of optics.
Diffraction gratings are used e.g. in spectrographs to provide angular dispersion of light. An important parameter of a diffraction grating is its efficiency, i.e. the fraction of the incoming light, in a particular wavelength bandwidth, that is diffracted towards a destination angle where e.g. a detector or further light guiding optics are placed. Diffraction gratings generally operate most efficiently with the transverse electric (TE) components of the electromagnetic waves impinging the grating, i.e. light wherein the electric field oscillates along the direction of the grating lines.
Higher efficiency for gratings is traditionally achieved by improving the quality of the grating, i.e. minimize stray light by minimizing roughness of the groove surfaces, optimizing the groove shape (e.g. blaze angle) to maximize the diffracted light in a certain order. For example, holographic methods can be used to produce gratings with a very uniform line density, therefore resulting in efficient diffraction behavior. Typically, the less uniform the line density, the wider the angular spread of the diffraction order and thus the less resolving power of the grating.
Alternatively, ruling methods can be used to produce sawtooth gratings. The line density typically is less uniform for ruled gratings than for holographically produced gratings. Ruled gratings typically suffer from “Rowland ghosts”, generated by the presence of several line densities on the same grating. An advantage of ruled gratings is an increased design freedom over sinusoidal gratings. By adjusting the angle of the ruling tip with respect to the sample, a particular blaze angle can be achieved in the sawtooth pattern, which may result in an improved efficiency over holographically produced gratings. Disadvantageously, ruled gratings are limited by the size and shape of the ruling tip and the precision of the ruling instrument. Further information on holographic and ruled gratings can be found e.g. in the “Diffraction Grating Handbook, sixth Edition” by Newport.
Fabini et al. (U.S. Pat. No. 6,449,096) disclose efficiency calculation of triangular gratings as a function of polarization, wavelength, blaze angle, grooves/mm, incident angle, triangle groove height, and back angle. Diffraction gratings are manufactured classically with the use of a ruling engine by burnishing grooves with a diamond stylus in a substrate or holographically with the use of interference fringes generated at the intersection of two laser beams or through a combination of photolithographical etching.
Chou et al. (U.S. 2008/0230947) disclose a method wherein a smooth nanoscale surface pattern is produced. This is achieved by providing a mold substrate crystalline material with a nanoscale pattern of etch resistant material and anisotropically etching the masked mold with a wet etchant having an etching rate in the <111> crystal plane slower than the <100> plane. A replica of the mold is produced by providing a work piece with a moldable surface and pressing together the mold and the work piece. In an embodiment line-uniformity is improved by a method wherein mask material is coated by shadow evaporation on both sides of a resist triangle in two consecutive deposition steps. According to Chou, grating efficiency is improved by the smoothness of the grating sidewalls. However, this improvement may be insubstantial on the overall scheme of efficiency improvements, in particular for a grating operation regime of interest as exemplified in the present disclosure.
There is yet a need for an easy to manufacture diffraction grating with controlled efficiency.